I think you should be focusing more on whether Cap can trust Vann as a crewmate rather then seeing him as a potential "threat" to your relationship. Cap needs help and he's glad to have someone around he thinks he can trust.The writing in the current update is the cap expressing the opinion of Vann being his bro and how much he likes him and is glad to have him around. It's really forced because if you don't feel that, it's playing a character you then develop distance from and.
I'm surprised to read that about not asking you to be bros... True it isn't asking, it's placed there with no option to disagree I guess. Anyway, I'm not currently seeing anything in the writing that hints at anything else there so maybe if you had a different intention with Vann you could hint at it?
Personally I'm getting really uncomfortable vibes about the whole thing. Really would not enjoy ending up stuck in a confined space with my partners ex. That just isn't my sort of adventure, I guess some wont and don't mind that though. I'd find that deeply unpleasant, and do here too...
Vann and Noel's past relationship can provide drama and good story-telling opportunities. Your insecurities as a person are dumb